Do this because retrying reading on higher levels (like the demuxer)
usually causes tons of problems. A hack like this is simpler and could
allow to remove some of the higher level retry behavior.
This works by trying to detect whether the file is appended. If we reach
EOF, check if the file size changed compared to the initial value. If it
did, it means the file was appended at least once, and we set the
p->appending flag. If that flag is set, we simply retry reading more
data every time we encounter EOF. The only way to do this is polling,
and we poll for at most 10 times, after waiting for 200ms every time.